The present invention relates to controlling the blending of a pair of coherent audio signals, such as the signals from two pickups on an electric guitar. It has been known that musically desireable effects derive from mixing the output of two pickups placed at different distances along the strings of the guitar. Further, it has been found that additional tonal variety is available by reversing the relative phase between the two pickup signals. It has become common to provide some form of phase switch on the body of the guitar to enable the musician to make such phase reversal at will. However, such a switch, even when used in conjunction with separate volume and tone controls for each pickup, places certain constraints on the freedom of the musician to smoothly and conveniently vary the blend between the two pickups in both of the possible phase relationships while playing the instrument. The phasing switch of necessity, causes an abrupt transition between the two phase conditions.